In a conventional head-up display (HUD) device for a vehicle, a light-emitted image such as vehicle information, which is displayed by a display unit, is projected onto a projection target such as a windshield of a vehicle so that the displayed image may be displayed as a virtual image. JP 2011-207431A (US 2001/0241596 A1) discloses one example of such a HUD device, in which a display image provided by a display unit is reflected by a reflection mirror such as a concave mirror and projected onto a projection target. By using the reflection mirror, the HUD device can be mounted in a limited space in a vehicle.
In this HUD device, a stepper motor is used to rotate the reflection mirror in response to a drive signal corresponding to an adjustment instruction from an external side so that a display position of virtual image may be adjusted. According to this configuration, a passenger (driver) in a vehicle is enabled to adjust a position of a virtual image of vehicle information to a position, which is more readily viewable, by inputting a position adjustment instruction to the HUD device.
In addition, in the HUD device, the stepper motor is continued to be driven by the drive signal until it is stabilized electrically, when the input of the adjustment instruction is stopped. If the drive signal is stopped at a point, which is not at the electric stabilization point, the stepper motor tends to step out of phase in response to an impact of external force or vibration. The drive signal is continued to be applied for a while to avoid this out-of-phase problem.
In the HUD device, it is assumed that plural electric stabilization points, at which the stepper motor is electrically stabilized by holding torque generated when powered, and plural mechanical stabilization points, at which the stepper motor is mechanically stabilized by detent torque provided when not powered, match each other. Under this assumption, the stepper motor is stabilized by continuously applying the drive signal until the electric stabilization point is attained, even when the drive signal is stopped after the electric stabilization point. In this case, the stepper motor is assumed to be stabilized by the detent torque generated at the electric stabilization point.
In actual products, however, the electric stabilization points and the mechanical stabilization points are different in phase due to manufacturing tolerance and the like. As a result, in a case that the drive signal is continued to be applied at the electric stabilization point and thereafter stopped, the stepper motor temporarily stops and then starts rotation toward the mechanical stabilization point. This causes the display position of the virtual image to move again after being stopped once. Passengers in the vehicle will thus feel discomfort and even unreliability of the vehicle information displayed as the virtual image.